In twin rotor screw compressors, the pressure gradient is normally in one direction during operation such that fluid pressure tends to force the rotors towards the suction side. The rotors are mounted in bearings at each end so as to provide both radial and axial restraint. The end clearance of the rotors at the discharge side is critical to sealing and the fluid pressure tends to force open the clearance. Also, the axial forces tend to drive the suction end of the rotors into the casing which can damage the rotors if contact between the rotor(s) and casing is allowed to occur.
Rolling element bearings capable of both axial and radial restraint are mounted to a shaft. Current practice dictates either an interference fit or key between the bearing and shaft to prevent relative rotation between the two components. The use of a key is more costly due to the requirement for additional parts (keys) and features (keyslots) which can also stack up tolerances for the various parts/features. The use of an interference fit makes assembly more laborious in that the bearing is secured to the shaft after the shaft is set in place but before the accuracy of the placement of the bearing is determined. Once secured in place, the bearing cannot be easily relocated to correct any error in its positioning since the interference fit is normally accomplished by pressing into place or by heating the bearing and letting it cool and shrink around the shaft. After determining the accuracy of placement, it may be necessary to relocate the interference fitted bearing(s) on the shaft(s) using a press and special tooling to properly locate the rotor(s) in the casing. Even with special equipment, relocation of interference-fitted bearings is difficult and uncertain at best.